1. Field of the Invention
The invention relates to hot conductors and somewhat more particularly to hot conductors composed of a sintered oxide mixture containing rare earth metals.
2. Prior Art
Resistor elements which possess a negative temperature coefficient of resistance are generally referred to as hot conductors. This type of resistor is also known as a thermistor or a NTC-resistor. Generally, such hot conductors are composed of sintered oxides of manganese, iron, cobalt, nickel, copper, zinc and mixtures thereof. Hot conductors composed of such metal oxides and mixtures thereof are typically not suited for high temperature applications because these metal oxides and mixtures thereof decompose at temperatures above about 600.degree. C. Further, since irreversible changes may occur within hot conductors of this type even at low temperatures, the prior art has generally limited their usages to maximum temperatures in the range of about 300.degree. to 350.degree. C.
"Zetschrift fuer Elektrochemie" (Journal for Electro-Chemistry) 1959, pages 269-274 suggests that the conductivity of rare earths increases with rising temperatures, however, no suggestions are made for incorporating rare earths or mixtures thereof in hot conductors.
In addition, the prior art is aware that hot conductors usable at higher or high temperatures may be produced from a mixture of a rare earth and zirconium oxide. For example, British Patent Specification No. 874,882 suggests a hot conductor comprised of a mixture of yttrium and zirconium oxide, while German Offenlegungsschrift No. 2,333,189 suggests a hot conductor comprised of a mixture of praseodymium and zirconium oxide. However, hot conductors composed of such materials exhibit a varistor effect, i.e., the resistance value of such hot conductors is dependent not only upon the temperature but also on the applied voltage.